New Horizons on the Nuclear Landscape

New calculations have quantified the boundaries and uncertainties of the ‘chart of the nuclides’—the extended periodic table of all matter.

Map of bound even-even nuclei as a function of the proton number Z and the neutron number N. There are 767 even-even isotopes known experimentally; both stable (black squares) and radioactive (green squares). Mean drip lines and their uncertainties (red) were obtained by averaging the results of different models. The two-neutron drip line of SV-min (blue) is shown together with the statistical uncertainties at Z=12, 68, and 120 (blue error bars). The S2n=2 MeV line is also shown (brown) together with its systematic uncertainty (orange). The inset shows the irregular behavior of the two-neuron drip line around Z=100.

The Science

Of the several thousand nuclides thought to exist, only around 3,000 have been observed, and only 288 are considered to be stable. Scientists typically map these nuclear species on a ‘chart of nuclides’. The boundaries marking the end of nuclear binding—where the addition of more protons or neutrons leads to a nuclide that is unbound—are known as drip lines, and until now have not been well known for the heavier elements. DOE researchers recently quantified the uncertainty of these drip lines using a technique known as nuclear density functional theory, providing a clearer picture of the map of stable isotopes.

The Impact

Quantifying the limits of nuclear binding is important for understanding the origin of elements in the universe. The astrophysical processes responsible for the generation of many heavy elements operate very closely to the drip lines; hence, the structure of very exotic, weakly bound nuclei directly impacts the way the elements are produced in stars.

Summary

There are 288 stable or nearly stable nuclei that occur in nature, comprising 99.9 percent of the matter in the visible world around us. Some 3,000 more have been synthesized in laboratories. These nuclear species have been mapped onto a chart of nuclides—the periodic table of the nuclear physics world. Until recently, the boundaries marking the edge of where nuclei can exist in this nuclear landscape—where the addition of one more proton or one more neutron will cause the nucleus to fall apart—has been highly uncertain, especially for heavier elements. Research using a technique known as nuclear density functional theory carried out at the University of Tennessee and ORNL using one of the world’s most powerful supercomputers now predicts that the number of bound nuclides with atomic numbers between 2 and 120 is around 7,000. These findings represent a major advance in our understanding of nuclear stability, and where the ultimate limits of nuclear existence occur. Understanding the stability of nuclides is important to many applications and to natural phenomena such as the stellar processes that create the matter around us.